The present invention relates to temperature-sensitive elements, and more particularly to elements suitable for use in making measurement probes capable of accurately determining the temperature of a fluid by measuring the electrical resistance of a coiled winding.
Such temperature-sensitive elements must be capable of operating over very wide temperature ranges (e.g. -250.degree. C. to +700.degree. C.) and/or in aggressive environments, while withstanding shock, vibration, pressure, and flow forces as well as possible. This applies in particular for space applications where measurement probes are used for determining the temperature of a flow of liquid hydrogen, or of liquid oxygen, or of any other liquefied gas.
The designers of such temperature-sensitive elements must thus cope with very difficult problems concerning insulation and mechanical strength. In addition, they seek to provide elements having response times which are as short as possible (e.g. less than 1 second), both for a rise and for a fall in temperature. Added to that is the need to be able to provide elements which are small in size, i.e. a few centimeters long and a few millimeters in diameter, thus making the problems to be solved even more difficult.
The state of the art is illustrated by temperature-sensitive elements of the type comprising an elongate ceramic mandrel along which at least two non-insulated connection wires (generally made of platinum) pass longitudinally and connect with a bifilar winding wound on the outside surface of said mandrel, with an insulating coating of glass surrounding the elongate mandrel and its bifilar winding.
Such temperature-sensitive elements are widely used, but they are nevertheless very vulnerable where the bare connection wires leave the insulating covering.
Proposals have been made to improve the installation of the bare platinum wires for the purpose of protecting them, by covering them with an insulating sheath of plastic material, e.g. polytetrafluoroethylene.
In addition to the temperature limitations of such a system (in general -50.degree. C. to +300.degree. C.), a major drawback remains in any event due to the presence of exposed zones of the connection wires where they leave the insulating coating, between the end edge of said coating and the edge of the protective sheaths.
There is always a "plane of weakness" at these exit points with a danger of breakage due to this zone being fragile and possibly being subjected to severe mechanical stresses (shock, vibration, pressure, flow forces), with the work hardening of the wires in any event deteriorating their electrical characteristics by modifying their resistivity. Naturally, these risks become even worse when the temperature-sensitive element is immersed in a conducting atmosphere.